1. Field of the Invention
The present subject matter is directed generally to orthopedic prostheses, joint replacement systems and methods and, more particularly, to a multi-axis mobile bearing ankle prosthesis.
2. Background Information
The concept of total ankle arthroplasty has a long and relatively unsuccessful history due to the high failure rate often associated with the original implant devices and implantation techniques. Only recently has total ankle arthroplasty regained some recognition as a viable treatment for limited indications and as a viable alternative to an ankle joint fusion, which is often referred to as the gold standard of treatment. It has been shown that replacement of an ankle joint with an ankle prosthesis can be particularly challenging due to the relatively small articular contact surfaces of the ankle, complex biomechanics of both the ankle and hindfoot joints, limited and risky access to the ankle joint during replacement, and wide variation in patient candidacy. Past flawed design rationale and the above factors have led to a high rate of post-operative complications such as loosening of the ankle prosthesis, subsidence, pain, abnormal ankle prosthesis wear, and/or meniscal/bearing breakdown—often leading to ankle implantation failure.
In addition to the technical difficulties, regulatory agencies have classified ankle prosthetics in a manner which is often viewed as substantially limiting scientific progress in the field of ankle replacement due to the financial burden of obtaining market clearance for such devices.
Currently, two classes of ankle prosthetics are generally available; a semi-constrained ankle prosthetic and an unconstrained ankle prosthetic. Both types of ankle prosthetics utilize either a three (3) piece and two (2) component design (with the meniscal portion/bearing locking into the tibial plate) or a three (3) piece and three (3) component design (with a mobile/unlocked bearing) including an upper, middle, and lower component (tibial, bearing, and talar component, respectively).
A semi-constrained ankle prosthesis typically provides a tibial fixation component (usually metal) which provides firm attachment to the distal end of the tibia bone. A talar component provides firm attachment to the superior surface of a prepared talus, and provides on its upper or proximal side a surface for articulation. A bearing component can fit between the tibial component and the talar component and is typically locked/fixed to the tibial component. The underside of the bearing can provide a surface to articulate with the surface of the talar component. These surfaces can be structured such that all motions present in a normally aligned ankle joint can be at least partially replicated. Such motions can include plantar/dorsiflexion, rotation about the tibial/talar axis, some medial/lateral translation, and some anterior/posterior translation. Rotations in the frontal plane or motion in the transverse plane are usually not well supported as there is little curvature in this region. The influence of the subtalar joint axis of motion is not generally taken into consideration with this type of the device, which can alter the function and position of the talar body and therefore the talar component. These motions can occur actively and lead to edge loading, causing higher stress and greater propensity for wear. Also, as the articular surfaces can be designed for mismatch, even under optimum implant positioning and loading, higher stress will be seen at the contact point due to the point loading associated with mismatched radii of the articular surfaces as the surface contact areas are smaller and thus experience much greater loads.
Unconstrained prosthetics are all generally the same in function. They are similar to semi-constrained prostheses except that the potential for motion between the tibial component and the bearing component is designed into the prosthesis with anterior to posterior rotation of the ankle in the sagittal plane and gliding motion in the transverse plane. There is not intimate fit between the bearing component and the tibial component as the tibial component usually has a flat undersurface and the bearing component usually has a simple flat upper surface so that translation and rotation are allowed at this interface. Further, the interface between the talar component and the bearing component can have a curvature that is matched, so there is a large contact surface area and optimized contact stress that can result in reduced wear. This matched articulation can be accomplished because other motions are allowed for between the tibial and bearing components. It has been clearly shown with clinical history in all joints that if these motions are not allowed for, the force must be absorbed at the implant bone interface, and can lead to a greater propensity for loosening. The current systems in this category do not often address the frontal plane motion influence of the underlying subtalar joint axis.
Therefore, it is apparent from the above that the need exists for a polyaxial endoprosthetic ankle joint replacement implant.
It is also apparent from the above that the need exists for a better semi constrained polyaxial endoprosthetic ankle joint replacement implant.